Point-to-multipoint or group communications can be defined as communications from a single source towards at least two recipients. In packet switched group communications, transmission and switching functions are achieved by packet-oriented techniques, so as to dynamically share network transmission and switching resources between a multiplicity of connections. In packet switched data transmission e.g. a voice message may be broken down into fixed-length addressed packets, which are then transmitted to their destination independently.
Group communications can be multicast communications, unicast communications or anycast communications. Anycast can be defined as transmission of a single protocol data unit (PDU) where the PDU reaches the nearest single destination, whereas multicast can be defined as transmission of a single protocol data unit where the PDU reaches a group of one or more destinations. Group communications using multicast results in economical use of resources, but the implementation requires complicated management of group multicast addresses, group membership and location of group members. The principal problem in this approach is that it is not possible to keep group information separate from the mobile network—the various elements in the mobile network need to be aware of group information.
IP (Internet Protocol) multicasting can also be defined as sending IP packets to a host group. Each group has its own IP address. Joining a group or departing the group is dynamic, i.e. a subscriber can join or depart the group any time, and there are practically no limitations on the size of the group. One host can belong to many groups, and a host sending packets to the group does not have to belong to that group. The host can connect to a group by IGMP (Internet Group Management Protocol) protocol.
Multicast communication works on top of TCP/IP (Transmission Control Protocol/Internet Protocol) based networks, where multicast routers guide the multicast transmission of IP packets. However, in multicasting data is not transmitted via routers to subnets where there are no subscribers to a service.
A third alternative described in a co-pending U.S. patent application Ser. No. 09/835,867, is to use individual, unicast sending to each group member separately by a group bridge provided in an IP network overlaying a radio network employed as an access network. The radio network controls the mobility and radio resources. However, this approach becomes problematic when multiple group members are located in the same cell. In case group members are closely concentrated in a location, this can prevent the voice from reaching some of the members due to congestion in the radio network. The group voice is usually listened to at loudspeaker volumes. Unicast has then also the problem that if two or more recipients happen to be very close to each other, the delay difference in the received voice heard by the recipients may severely limit the intelligibility of reception.